Study Notes: Prisms

A problem with normal glass-fronted mirrors is that part of the light
is reflected from the glass front and the remaining refracted light is
laterally displaced when it emerges back through the glass from the polished
metal reflector.

This causes multiple images to appear. Normally we cannot see these
multiple images because they are very close together. However, a sensitive
scientific instrument may be able to detect the separate light beams
and this can lead to a distortion of measured results.

Special mirrors that have the silvered reflective material on the front,
instead of the back, of the glass can be used to overcome this effect.
With these mirrors there is no protective layer over the reflecting surface
and as a result there is the problem that damage may occur during cleaning
of the reflective surface. To overcome this, special dust free instrument
compartments must be used. Another problem is that air will corrode the
metallic surface and this will distort the specular reflection.

An easier approach is to use glass prisms to do the reflecting.
A prism can be used to reflect light with little or no distortion,
in two ways.

The incident ray enters the surface of the prism at a 90° angle and
at this angle it is not refracted but passes straight into the glass.
When the light tries to emerge from the right hand side of the glass it
cannot. This is because it strikes the inner surface of the glass at a
45° angle. This is larger than the critical angle (41.5° for the
glass/air) so the light is totally internally reflected at a 45° angle.
One reflection inside the prism changes the direction of the light beam
by 90° while two reflections changes the direction by a total of 180°.